Titanium oxide has been known as a typical photo-oxidation catalyst, and hitherto applied to an antibacterial agent, an antifogging agent that utilizes their super hydrophilicity, or the like. There have been known three types of crystal structures for titanium oxide; anatase, rutile and brookite. Of these types, anatase is considered to have the highest photocatalytic activity. In recent years, however, it has been known that the photocatalytic activity is further enhanced by bringing a rutile-type component partially into contact with an anatase-type component and mixing them. For that reason, there has been proposed that ultrafine particles of a rutile-type titanium oxide are mixed with an anatase-type titanium oxide. It has also been expected that high activity is exhibited by using ultrafine particles of a rutile-type titanium oxide alone. However, since in the conventional vapor phase method, a rutile-type titanium oxide is treated at a high temperature, its particle diameter becomes large due to sintering. Thus, there has been a drawback such that the photocatalytic activity is consequently reduced due to the decrease in the surface area.
On the other hand, there has been reported a method for synthesizing a rutile-type titanium oxide using the low temperature wet prosess by H. D. Nam et al. (Non-patent Document 1). However, in this method, there is also the problem of decrease in the photocatalytic activity because there is formed an aggregate having a particle diameter of 200 to 400 nm in which a long-fibrous rutile-type titanium oxide is gathered. In order to overcome these drawbacks, it is also provided highly dispersive rutile ultrafine particles useful for high refractive index materials or ultraviolet absorbing materials requiring transparency.
Non-patent Document 1: Jpn. J. Appl. Phys., Vol. 37, p. 4603 (1998)